Low temperature dehydrogenation properties of ammonia borane within carbon nanotube arrays: a synergistic effect of nanoconfinement and alane

Ammonia borane (AB, NH3BH3) is considered as one of the most promising hydrogen storage materials for proton exchange membrane fuel cells due to its high theoretical hydrogen capacity under moderate temperatures. Unfortunately, its on-board application is hampered by the sluggish kinetics, volatile byproducts and harsh conditions for reversibility. In this work, AB and AlH3 were simultaneously infiltrated into a carbon nanotube array (CMK-5) to combine the synergistic effect of alane with nanoconfinement for improving the dehydrogenation properties of AB. Results showed that the transformation from AB to DADB started at room temperature, which promoted AB to release 9.4 wt% H2 within 10 min at a low temperature of 95 °C. Moreover, the entire suppression of all harmful byproducts was observed.


Introduction
Large-scale applications of hydrogen in proton exchange membrane fuel cells (PEMFCs) are greatly promoted by the booming developments of efficient hydrogen storage technology. 1 Hydrides composed of light elements are an option due to their high volumetric storage capacities and compact nature of hydrogen storage. 2,3 AB is a promising light weight hydride because of its remarkable hydrogen capacity (19.6 wt% and 140 g L À1 ), moderate desorption temperatures and relatively high air stability. [4][5][6] Upon thermal decomposition, the liberation of H 2 from AB takes place in three steps (eqn (1)-(3)): [7][8][9] nNH 3 BH 3 / (NH 2 BH 2 ) n + nH 2 (90-120 C, 6.5 wt%) (1) (NH 2 BH 2 ) n / (NHBH) n + nH 2 (120-200 C, 6.9 wt%) (2) (NHBH) n / nBN + nH 2 (>500 C, 7.3 wt%) Owing to the extreme high desorption temperature of step (3) (>500 C), only the hydrogen release from eqn (1) and (2) below 200 C are regarded as possible for practical applications in combination with fuel cell systems. For PEMFCs, however, there are still three main challenges involved in the rst two steps: (a) the emission of detrimental byproducts (i.e., ammonia, diborane, and borazine); (b) relatively high desorption temperature (100-200 C); (c) poor reversibility. To overcome these obstacles, several strategies have been explored, including the inltration of AB in porous structures (nano-connement), 10-12 the addition of metal based catalysts, [13][14][15] modifying thermodynamics through chemical alteration, [16][17][18] etc. Among them, nanoconnement is an effective approach to positively affect the dehydrogenation thermodynamics and kinetics of AB, which originates from the effects of increased surface energy, induced defects and vacancies as well as shortened diffusion distances. 19 The pioneering work by Gutowska et al. demonstrated that the decrease of desorption temperature and the suppression of borazine were simultaneously achieved by the capsulation of AB into mesoporous silica. 10 However, they didn't mention the problem of NH 3 liberation during the decomposition process, which would severely poison PEMFCs even at a very low level (1 ppm). 20 This issue could be solved by the combination of nanostructure connement with metal catalysts. Li et al. incorporated AB into a lithium (Li) functionalized ordered mesoporous carbon framework (CMK-3), and considerable enhancement of dehydrogenation properties as well as complete suppression of all volatile byproducts could be observed in the AB/Li-CMK-3 composite. 21 Since then, various surface functionalized nanoscaffolds have been explored, including carbon nanotubes, [22][23][24] metal-organic frameworks, 25-27 mesoporous 3D boron nitride, 28 Pd/halloysite nanotubes, 29 etc. On the other hand, the destabilization of AB and suppression of volatile byproducts could also be achieved by compositing with metal hydrides (MH). [30][31][32][33] Kang et al. found that the mechanically milled mixture of AB and MgH 2 could release 8 wt% H 2 within 4 h at around 100 C without the byproducts of diborane and borazine. 31 In AB-MH systems, Nakagawa et al. demonstrated that Pauling electronegativity of M is good to indicate the amount of undesired byproducts (NH 3 and B 2 H 6 ), and the level of NH 3 decreased with the increase of electronegativity. 32 Therefore, AlH 3 is more effective than MgH 2 on inhibiting the emission of NH 3 due to the higher electronegativity of Al. Compared with nanoconned systems, the dehydrogenation temperatures of these MH-doped composites are relatively high (>100 C).
These observations and advancements imply that further improvements of the dehydrogenation properties of AB can be expected through the combination of nanoconnement and MH. CMK-5, an ordered mesoporous carbon material possessing very high specic surface area (up to 2000 m 2 g À1 ) and bimodal porosity 34 (much higher than that of SBA-15 (ref. 35) and CMK-3 (ref. 36)), is a perfect host material for fabricating of nanoconned materials. [37][38][39] However, to our knowledge, CMK-5 has never been used as nanoscaffold for hydrogen storage materials before. Therefore, the composites of AB and AlH 3 were encapsulated into CMK-5 in this work, and the possible improved mechanism of dehydrogenation properties was proposed.

Experimental section
Materials synthesis CMK-5 was prepared according to the literature. 38 Alane was synthesized by the organometallic method. 40,41 Ammonia borane (97 wt%) purchased from Sigma-Aldrich was used directly without further purication. Before use, CMK-5 was degassed for 20 h at 160 C to remove adsorbed moisture and gases. Composites of x wt% AB (x ¼ 20, 30, 40, 50) and CMK-5 with different percentages of AB in CMK-5 were denoted as xAB@CMK-5, and composites of 30 wt% AB and x wt% AlH 3 (x ¼ 10, 15, 30) in CMK-5 were denoted as (30AB : xAlH 3 )@CMK-5. Schematic illustration of the preparation process of (30AB : xAlH 3 )@CMK-5 is shown in Fig. 1. AB and AlH 3 was physically mixed in a mortar by hand-milling and then dissolved in anhydrous THF at room temperature. Aerwards, the THF solution of AB and AlH 3 was instilled into CMK-5, and the resulted suspension was further stirred for 2 h at room temperature. Finally, the solvent was evaporated under vacuum overnight. A physical mixture of same amount of AB and AlH 3 (denoted as AB/AlH 3 ) was also prepared for comparison. All preparation procedures were carried out under puried argon atmosphere in a glove box or using standard Schlenk technique.

Characterization
Wide angle X-ray diffraction (XRD) patterns were recorded on a STOE STADI P diffractometer in Debye-Scherrer transmission geometry with Cu/Ka radiation, and all samples were sealed into glass capillaries with a diameter of 0.5 mm for measurements. Small angle XRD patterns were collected on a Stoe q-q diffractometer in Bragg-Brentano geometry with Cu/Ka radiation. The nitrogen adsorption-desorption curves were measured on a NOVA 4200e instrument at À196 C. All these samples were degassed under vacuum for 24 h at room temperature before measurements. The BET surface areas were obtained from the data in the relative pressure range of 0.05 and 0.20. The pore volumes and pore size distributions were calculated from the desorption branches of isotherms by employing the Barrett-Joyner-Halenda (BJH) model. High-resolution scanning electron microscope (HRSEM) images, and transmission electron microscope (TEM) images were operated on a Hitachi S-5500 ultrahigh-resolution cold eld emission scanning microscope. Thermogravimetric analysis (TGA) and  differential scanning calorimeter (DSC) measurements were performed on a TGA/DSC STAR e system with a heating rate of 5 C min À1 under Ar atmosphere. The released gas was analyzed using mass spectrometry (MS, HPR-20 QMS) with an argon purge rate of 50 mL min À1 . IR-spectra were measured with a Fourier transform infrared spectroscopy (FT-IR, Nicolet 560). Solid-state 27 Al nuclear magnetic resonance (NMR) spectroscopy was recorded on a Bruker Avance 500WB spectrometer, using a Doty CP-MAS probe with no probe background. X-ray photoelectron spectroscopy (XPS) measurements were conducted on a Kratos HSI spectrometer with a hemispherical analyzer. The monochromatized Al K a X-ray source (E ¼ 1486.6 eV) was conducted at 15 kV and 15 mA. An analyzer pass energy of 40 eV was adopted for the narrow scans. The hybrid mode was employed as lens mode. The base pressure in analysis chamber was 4 Â 10 À7 Pa during the process. The values of binding energy were referred to the C 1s peak (284.5 eV) with the purpose of accounting for charging effects.

Results and discussion
The structure of as-prepared CMK-5 was characterized by small angle XRD, N 2 adsorption-desorption isotherm, SEM and TEM, and the results are presented in Fig. 2. Small angle XRD measurement (Fig. 2a) gives peaks of (100), (110), (200), (210) and (300) reections, indicating the ordered 2D hexagonal structure of CMK-5. 35 N 2 adsorption-desorption analysis (Fig. 2b) shows that the BET surface area and pore volume of CMK-5 are 1650 m 2 g À1 and 1.69 cm 3 g À1 , respectively. The typical structure of CMK-5 with bimodal porosity is proved by the two maxima of pore size distribution (inset image of Fig. 2b). SEM images ( Fig. 2c and d) reveal the rod-like morphology of CMK-5, and further SEM images of cross section ( Fig. 2e and f) as well as TEM images (Fig. 2g and h) conrm its uniform and ordered 2D hexagonal structure.
AB and/or AlH 3 were loaded into CMK-5 by the infuse method, and the impregnation of them were conrmed by the XRD, FT-IR and N 2 adsorption-desorption analysis. Fig. 3a shows the XRD patterns of CMK-5, 30AB@CMK-5, AB/AlH 3 and (30AB : 30AlH 3 )@CMK-5. Compared with the obvious diffraction peaks of AB and AlH 3 in the AB/AlH 3 composite, these peaks become hardly observable aer encapsulating into CMK-5 frameworks, indicating the dispersion of AB and AlH 3 into mesoporous CMK-5 at a nano or amorphous state. As shown in  Fig. 4a and b show the N 2 adsorption-desorption isotherms and pore size distributions of CMK-5, 30AB@CMK-5 and (30AB : 30AlH 3 )@CMK-5, respectively. Compared with pure CMK-5, the specic surface areas and pore volumes are declined dramatically from 1650 to 501 and 328 m 2 g À1 , and from 1.69 to 0.53 and 0.28 m 3 g À1 , respectively, for 30AB@CMK-5 and (30AB : 30AlH 3 )@CMK-5 (Fig. 4a). The corresponding pore size distributions also show a gradual loss of two maxima (Fig. 4b), further conrming that most of the pores of CMK-5 are lled or blocked by AB and/or AlH 3 .
To further assess the distribution of AB and/or AlH 3 , these conned samples are also characterized by SEM, and the results are shown in Fig. 5. In the pristine CMK-5 (Fig. 5a), the typical pore channels can be observed on the surface, while most of these pore channels in 30AB@CMK-5 (Fig. 5b) are occupied aer the impregnation of AB. For (30AB : 30AlH 3 )@CMK-5 (Fig. 5c), no obvious channel feature of CMK-5 can be identi-ed due to the accumulation of AB and AlH 3 . Further energy dispersive X-ray spectroscopy (EDS) mapping results of the surface (Fig. 5d-f) and cross section (Fig. 5g-i) of (30AB : 30AlH 3 )@CMK-5 indicate the well distribution of AlH 3 Fig. 4 (a) N 2 adsorption-desorption isotherms at À196 C and (b) the pore size distributions of CMK-5, 30AB@CMK-5 and (30AB : 30AlH 3 ) @CMK-5.  This journal is © The Royal Society of Chemistry 2020 RSC Adv., 2020, 10, 19027-19033 | 19029 both in and outside the channels of CMK-5. However, we failed to track the distribution of B and N through EDS due to their light nature and relatively low loaded amount here. In fact, AB was supposed to be well distributed in the inner and external surfaces of CMK-5 through the impregnation method. 21,23,40 To further observe the distributions of AB and AlH 3 in (30AB : 30AlH 3 )@CMK-5, TEM was also conducted and the results are presented in Fig. 6. As shown in Fig. 6a, (30AB : 30AlH 3 )@CMK-5 generally displays a rather homogeneous structure. An enlargement of the dotted circle a 1 shows that there are some aggregated particles outside the pore channels (Fig. 6b), which was conrmed to be AlH 3 by EDS result. Further enlargements of the dotted circle a 2 (Fig. 6d and e) show that these AlH 3 particles with a size of 20-40 nm stay in a highly crystalline state. While from the high-resolution TEM image of the dotted circle a 3 (Fig. 6c), no obvious AB or AlH 3 particles can be distinguished from the frameworks of CMK-5, indicating the possible amorphous structure of AB and AlH 3 . In combination with the SEM and TEM results, the schematic diagram of the actual structure of (30AB : 30AlH 3 )@CMK-5 is shown in Fig. 6f. The majority of AB and AlH 3 uniformly distribute both in and outside the channels of CMK-5, while a small part of AlH 3 gather to form large particles on the surface.
MS, TGA, DSC and isothermal measurements were conducted to compare the dehydrogenation behaviors of AB, 30AB@CMK-5, AB/AlH 3 and (30AB : 30AlH 3 )@CMK-5 (Fig. 7). All these data have been normalized to the loaded percentage of AB. As shown in Fig. 7a, the decomposition of neat AB displays a two-step process at 117 C and 162 C to release the rst and second equivalent of hydrogen. Meanwhile, several byproducts (i.e., ammonia, diborane and borazine) are detected in accompany with the liberation of hydrogen above 100 C. The connement of AB into CMK-5 contributes to considerable enhancements in the dehydrogenation behavior with an onset dehydrogenation temperature as low as 50 C and two decomposition peaks at 98 C and 140 C. Moreover, a signicant decrease of ammonia and a complete suppression of diborane and borazine are observed. These improvements are correlated with the nanoconnement effect of CMK-5, which affects the decomposition thermodynamics and kinetics of AB. 41 Notable reductions of dehydrogenation temperature and byproducts are also observable in AB/AlH 3 . However, the liberation of NH 3 is still detectable during the heating process of 30AB@CMK-5 and AB/AlH 3 . Further inhibition of NH 3 can be realized through the synergistic effect of CMK-5 and AlH 3 . As shown in Fig. S1, † the level of NH 3 decreases with increasing the amount of AlH 3 in (30AB : xAlH 3 )@CMK-5 (x ¼ 10, 15,30). A complete suppression of NH 3 can be achieved by loading the same amount (30 wt%) of AB and AlH 3 into CMK-5. As shown in Fig. 7b, the weight loss of 30AB@CMK-5, AB/AlH 3 and (30AB : 30AlH 3 )@CMK-5 are obviously lower than neat AB, further indicating the suppression of byproducts. Because of partly liberated NH 3 , the weight loss from 30AB@CMK-5 is much higher than that of (30AB : 30AlH 3 ) @CMK-5 under the same conditions. Fig. 7c shows the DSC curves of these samples. During the heating process, pure AB has a melting point at 108 C before its decomposition, while this endothermic peak cannot be detected in AB@CMK-5, AB/ AlH 3 and (30AB : 30AlH 3 )@CMK-5. This indicates that the rst equivalent H 2 is released prior to the melting of AB in these   samples. 42 On the basis of integral calculation from DSC curves, the decomposition reaction enthalpies of neat AB and AB/AlH 3 were calculated to be À19.9 and À19.7 kJ mol À1 , respectively, which is in agreement with reported values (À21 kJ mol À1 ) for pure AB. 10,21,43 The decomposition reaction enthalpies of AB in 30AB@CMK-5 and (30AB : 30AlH 3 )@CMK-5 increase to À2.9 and À2.0 kJ mol À1 , respectively, due to the destabilization effect of nanoconnement. Fig. 7d shows the isothermal dehydrogenation curves of (30AB : 30AlH 3 )@CMK-5 at different temperatures. For comparison, the dehydrogenation curves of neat AB at 85 C and 30AB@CMK-5 at 65 C were also presented. Even at low temperatures of 65 C and 55 C, AB in (30AB : 30AlH 3 ) @CMK-5 can liberate 2.3 wt% and 1.2 wt% H 2 within 10 min. Increasing temperature to 85 C, a capacity of 6.8 wt% H 2 can be released in 10 min while only a very small amount of hydrogen (0.2 wt%) is detected from neat AB within the same time. At temperature up to 95 C, the amount of released hydrogen in 10 min increases to 9.4 wt% and reaches 10.5 wt% in 30 min. These results indicate a kinetic improvement of the decomposition process of AB through the synergistic effect of nano-connement and alane.
To understand the decomposition mechanism, solid-state 11 B NMR and XPS spectra of 30AB@CMK-5 and (30AB : 30AlH 3 )@CMK-5 before and aer heating to 80 C and 100 C were recorded, and the results are presented in Fig. 8 and 9. As shown in Fig. 8a, the spectrum of 30AB@CMK-5 at room temperature shows one resonance peak at À25.3 ppm for unreacted AB, and two additional peaks at À15.1 and À39.3 ppm for BH 2  ]. DADB is a reactive product, the production of which is a crucial nucleation event that can trigger the rapid release of hydrogen, but its formation in pure AB required a heating period of 30 min at 85 C. 44 In our case, the transformation from AB to DADB starts in 30AB@CMK-5 during the impregnation or drying process at room temperature, demonstrating the destabilization effect of nano-connement on AB. Usually nanoscaffolds containing reactive oxygen functional groups can react with AB during the heating process. 10,21,45 XPS results also show hydroxyl groups in CMK-5 (Fig. S2 †). Hence, the formation of organic borates (-OBX) at 1.1 and 14.9 ppm could be the result of the reaction between AB and surface hydroxyl groups, 46 resulting in the cleavage of B-N and B-H bonds of AB to release NH 3 and H 2 during the decomposition process. 21 Aer heating up to 80 C, the resonances of AB and DADB at À15.1, À25.3 and À39.3 ppm obviously broaden, and the intensity signicantly decreases. Aer increasing the temperature to 100 C, these two starting materials are almost consumed and disappeared. A broad peak appears at about 20 ppm, which is a result of the formation of polyaminoborane (PAB) species aer the release of one equivalent of H 2 from AB. No borazine compound can be observed at 31.1 ppm. 47 For (30AB : 30AlH 3 )@CMK-5, as shown in Fig. 8b, the resonance peaks at room temperature are similar to 30AB@CMK-5, while the spectra at 80 C and 100 C are quite different. Aer heating from room temperature to 100 C, the peak intensity of AB gradually decreases while that of DADB stays almost unchanged with only a slight shi of the BH 4 position from À37.6 ppm to À40.2 ppm, suggesting that the addition of AlH 3 polymerizes DADB. Fig. 9 shows the B 1s and N 1s XPS spectra of 30AB@CMK-5 and (30AB : 30AlH 3 )@CMK-5 at different temperatures. The peaks at $189.5 eV correspond to B-O bond, which is close to a structure with oxidized trigonal geometry (BC 2 O, 190.0 eV), 48 further conrming the reaction between AB and surface hydroxyl groups. Aer thermal decomposition at 100 C, the peak associated with B-H bond appears at $185.2 eV in 30AB@CMK-5 (Fig. 9a) while not in (30AB : 30AlH 3 )@CMK-5 (Fig. 9b). Besides, there is an increase in the intensity of B-O bond both in 30AB@CMK-5 and (30AB : 30AlH 3 )@CMK-5. From the XPS elemental composition analysis, as shown in Table 1, obvious reduction of N content from 3.13 to 1.44 wt% is detected in 30AB@CMK-5 aer heat treatment due to the release of NH 3 . While there is almost no change of the N content in (30AB : 30AlH 3 )@CMK-5 under the same condition, this demonstrates the immobilization of N in this composite. These results indicate that, on the one hand, AB reacts with surface located functional groups to form organic borates (-OBX), and further addition of AlH 3 would change the decomposition route of conned AB in (30AB : 30AlH 3 )@CMK-5 because of coulombic attraction between the hydridic H dÀ of AlH 3 and protonic H d+ of NH 3 moieties in AB. 32,49 On the other hand, the hydridic H dÀ of AlH 3 and protonic H d+ of NH 3 moieties in DADB also results in the polymerization of DADB as   Fig. S5b †). This indicates that DADB undergo a structural modication instead of thermal decomposition due to the addition of AlH 3 . These results are consistent with the NMR results above. According to the abovementioned analysis, the decomposition pathways of 30AB@CMK-5 and (30AB : 30AlH 3 )@CMK-5 can be illustrated in Fig. 10. As shown in Fig. 10a, the decomposition of 30AB@CMK-5 can release hydrogen in two different ways: (1) AB particles on close contact with the surface of CMK-5 decompose via the reaction between -BH 3 and -OH to generate -OBH 2 species, resulting in the cleavage of B-N and B-H bonds to liberate H 2 and NH 3 ; (2) AB molecules inside CMK-5 channels would release H 2 through the pathway from DADB to PAB, PIB and H 2 because of the nanoconnement effect of CMK-5. As shown in Fig. 10b, the attraction between AlH 3 and NH 3 in (30AB : 30AlH 3 )@CMK-5 could inuence these two decomposition steps as follows: (1) coulombic attraction between the hydridic H dÀ of AlH 3 and the protonic H d+ of NH 3 moieties in AB would result in the cleavage of B-H and N-H bonds to prompt the production of -OBNH 2like structures, which is responsible for the immobilization of N and thus the suppression of NH 3 ; (2) hydridic H dÀ of AlH 3 reacts with active DADB to polymerize as -NH 2 BH 2 NH 2 BH 4species, leading to the stabilization of BH 4 during the heating process.

Conclusions
In summary, a new AB and AlH 3 nanostructure nanoconned composite, (30AB : 30AlH 3 )@CMK-5, has been fabricated for the possible applications in combination with low temperature PEMFCs. Results demonstrated that the synergetic effect of nanoconnement of CMK-5 and AlH 3 in (30AB : 30AlH 3 )@CMK-5 dramatically improve the dehydrogenation thermodynamics and kinetics of AB, meanwhile suppress the liberation of byproducts. AB in this framework composite can release 9.4 wt% H 2 within 10 min at a low temperature of 95 C, and the dehydrogenation enthalpy increases to À2.0 kJ mol À1 from the pristine value of À19.7 kJ mol À1 . This advancement demonstrates that the combination of nanoconnement with MH could be further explored to improve the hydrogen storage properties of AB and/or other hydride systems.

Conflicts of interest
There are no conicts to declare.